CLASSIFICATION OF CRUSTACEA. 325 



CLASS. I. CRUSTACEA. 



Arthropods breathing by gills situated on the legs, or respiring through 

 tJie body-walls. Body in the higher forms divided into two regions, a 

 cepJialo-thorax and abdomen. Two pairs of antenna ; mandibles usu- 

 ally with a palpus. Heart nearly square, or in the lower forms tubular. 

 Often a distinct metamorphosis. Sexes distinct, except in a few cases 

 (certain barnacles, etc.}. 



Subclass 1. Neocarida. Genuine Crustacea, with two pairs of anten- 

 nae; biting mouth parts ; mostly modern types. 



Order 1. Cirripedia. Sessile often retrograded ; antennae not devel- 

 oped, living parasitically, the appendages of the head some- 

 times forming net-like organs. Young hatched in the nau- 

 plius state. Suborder 1. Rhizocephala (Sacculina, Pelto- 

 gaster). Suborder 2. Genuine Cirripedia (Balanus, Lepas.) 



Order 2. Entomostraca. A cephalo-thorax developed ; mandibles and 

 three pairs of maxillae ; five pairs of thoracic feet, no ab- 

 dominal feet ; without any gills. The parasitic forms more 

 or less modified in shape, with sucking mouth-parts ; all 

 the young of the nauplius form. Suborder 1. Copepoda 

 (Cyclops). Suborder 2. Siphonostoma (Lernaea, Caligus, and 

 Argulus). 



Order 3. Branchiopoda. Thoracic feet leaf-like ; one to three pairs of 

 maxillae ; number of body-segments varying from a few to 

 sixty ; cephalo-thorax often well developed, and forming a 

 bivalved shell. Young usually a Nauplius. Suborder 1. 

 Ostracoda (Cypris). Suborder 2. Cladocera (Daphnia). Sub- 

 order 3. Phyllopoda (Limnadia, Apus, Branchipus, and Ar- 

 temia.) 



Order 4. Phyllocarida. Body compressed ; rostrum distinct from the 

 carapace ; thoracic feet leaf-like ; no metamorphosis. (Ne- 

 balia.) 



Order 5. Tetradecapoda. No cephalo-thorax, thoracic segments dis- 

 tinct ; respiration often carried on by the abdominal feet. 

 Suborder 1. Isopoda (Idotaea, Asellus). Suborder 2. Am- 

 phipoda (Gammarus). 



Order 6. Stomapoda. Abdominal feet respiratory. (Squilla.) 



Order 7. Decapoda. Cephalo-thorax well marked, abdomen often bent 

 beneath the cephalo-thorax ; breathing by gills attached to 

 the maxillipedes and legs. Heart often nearly square. 

 Usually a well-marked metamorphosis ; young called a 



